Co-culture-based biological carbon monoxide conversion by Citrobacter amalonaticus Y19 and Sporomusa ovata via a reducing-equivalent transfer mediator

The biological conversion of carbon monoxide (CO) has been highlighted for the development of a C1 gas biorefinery process. Despite this, the toxicity and low reducing equivalent of CO uptake make biological conversion difficult. The use of synthetic co-cultures is an alternative way of enhancing the performance of CO bioconversion. This study evaluated a synthetic co-culture consisting of Citrobacter amalonaticus Y19 and Sporomusa ovata for acetate production from CO. In this consortium, the CO2 and H-2 produced by the water-gas shift reaction of C. amalonaticus Y19, were utilized further by S. ovata. Higher acetate production was achieved in the co-culture system compared to the monoculture counterparts. Furthermore, syntrophic cooperation via various reducing equivalent carriers provided new insights into the synergistic metabolic benefits with a toxic and refractory substrate, such as CO. This study also suggests an appropriate model for examining the syntrophic interaction between microbial species in a mixed community

Small Current but Highly Productive Synthesis of 1,3-Propanediol from Glycerol by an Electrode-Driven Metabolic Shift in Klebsiella pneumoniae L17

Electrofermentation actively regulates the bacterial redox state, which is essential for bioconversion and has been highlighted as an effective method for further improvements of the productivity of either reduced or oxidized platform chemicals. 1,3-Propanediol (1,3-PDO) is an industrial value-added chemical that can be produced from glycerol fermentation. The bioconversion of 1,3-PDO from glycerol requires additional reducing energy under anoxic conditions. The cathode-based conversion of glycerol to 1,3-PDO with various electron shuttles (2-hydroxy-1,4-naphthoquinone, neutral red, and hydroquinone) using Klebsiella pneumoniae L17 was investigated. The externally poised potential of -0.9 V vs. Ag/AgCl to the cathode increased 1,3-PDO (35.5 +/- 3.1 mm) production if 100 mu m neutral red was used compared with non-bioelectrochemical system fermentation (23.7 +/- 2.4 mm). Stoichiometric metabolic flux and transcriptional analysis indicated a shift in the carbon flux toward the glycerol reductive pathway. The homologous overexpression of glycerol dehydratase (DhaB) and 1,3-PDO oxidoreductase (DhaT) enzymes synergistically enhanced 1,3-PDO conversion (39.3 +/- 0.8 mm) under cathode-driven fermentation. Interestingly, a small current uptake (0.23 mmol of electrons) caused significant metabolic flux changes with a concomitant increase in 1,3-PDO production. This suggests that both an increase in 1,3-PDO production and regulation of the cellular metabolic pathway are feasible by electrode-driven control in cathodic electrofermentation

Electron uptake from solid electrodes promotes the more efficient conversion of CO2 to polyhydroxybutyrate by using Rhodobacter sphaeroides

Microbial electrosynthesis (MES) is a promising strategy for the conversion of CO2 to useful chemicals. Nevertheless, the characteristics of electrode-associated cells in MES and their metabolic pathway regulation in CO2 fixation have not been elucidated. This study examined the electrode-driven polyhydroxybutyrate (PHB) production from CO2 in Rhodobacter sphaeroides. The electron uptake and regulation of the metabolic pathways differed in electrode-associated and suspended R. sphaeroides. The electrode-associated cells produced PHB at concentrations up to 23.50 \ub1 2.8% of the dry cell weight (DCW), whereas the suspended cells grew faster but with a lower cellular PHB content. Gene expression analyses showed that phaA expression was upregulated in electrode-associated R. sphaeroides, whereas phaB expression was downregulated in suspended cells. The electrode-associated cells expressed unconventional CO2 fixation enzymes, such as isocitrate dehydrogenase and formate dehydrogenase, with more PHB synthesis. These results show that CO2 can be upcycled to polymeric substances and provide novel insights into the genetic regulation of electrode-associated cells in MES